Calendered products

The ebonite compound before cure is a rather soft plastic mass which may be extruded, calendered and moulded on the simple equipment of the type that has been in use in the rubber industry for the last century. In the case of extruded and calendered products vulcanisation is carried out in an air or steam pan. There has been a progressive reduction in the cure times for ebonite mixes over the years from 4-5 hours down to 7-8 minutes. This has been brought about by considerable dilution of the reactive rubber and sulphur by inert fillers, by use of accelerators and an increase in cure temperatures up to 170-180°C. The valuable effect of ebonite dust in reducing the exotherm is shown graphically in Figure 30.3. 

The product quality, however, is related to the calendering process. Thickness gradients across the calendered product, thickness variations along the product, as well as cord density distribution determine the quahty of the product and are tremendously related to the process parameters. 

From the diagrams in Figures 35.34 through 35.40 it might become clear that all parameters will have a tremendous influence on the calendering process and therefore on the calendered product. The calender line speed seems to have the least influence on the calendering performance. 

High pressures can be found in the calender nip region. The resulting force acts on the adjacent calender rolls and causes a deflection of the rolls and therefore a thickness gradient across the calendered product. The total displacement Y(z) of the roll surface caused by roll deflection due to this viscous force can be approximated by Kopsch. ... 

If the acting force causes a roll deflection which exceeds a certain lateral displacement of the roll the thickness gradient across the calendered product might be beyond the required value which means that a calender shall include some special devices to compensate this roll deflection. Some well-known devices are... 

Factice has a number of uses in rubber compounds where it can stabilise dimensions of products such as hoses and tubing during the early stages of heating of the vulcanisation cycle. It imparts stability to extruded products, a silkiness to calendered products, gives good dimensional stability and overcomes many of the problems of crows feet and blistering. 

Control of product gauge is of considerable importance for the majority of products produced from calenders and various means have been used to achieve this from the early days of the machine. Inaccuracies in gauge on calendered products can not only mean out of specification goods being produced, but also one can be giving away free of charge considerable quantities of expensive compounds (unless the product is sold by weight, of course). 

After the reaction is completed, the suspension is transferred to a degassing tank, where steam is used to strip out the unreacted VCM. The PVC, now in a slurry with PVA, is separated by centrifuging and drying. The PVC powder or granules are then ready for additives and plasticizers for fabrication into one of the three mediums in which its used calendered products, extrusion, and molded products. 

The following factors have a direct bearing on the uniformity of the calendered products such as sheet, topped or frictioned fabrics. 

The thickness of the calendered product must be uniform in both the machine and cross-machine directions. Any variation in gap size due to roll dimensions, setting, thermal effects, and roll distortion due to high pressures developing in the gap, will result in product nonuniformity in the cross-machine direction. Eccentricity of the roll with respect to the roll shaft, as well as roll vibration and feed uniformity, must be tightly controlled to avoid nonuniformity in the machine direction. A uniform empty gap size will be distorted in operation because of hydrodynamic forces, developed in the nip, which deflect the rolls. The resulting product from such a condition will be thick in the middle and thin at the edges, as shown in Fig. 15.2. 

Special methods of incorporation processing history has an essential effect on conductivity shear imposed during mixing causes a fracture of secondary carbon aggregates increased temperature during mixing may preferentially form rubber-carbon bonds rather than the carbon-carbon bonds required for conductivity vulcanization temperature may affect recovery of broken connections between carbon-carbon bonds talc reduces melt viscosity which results in a smooth surface of extruded and calendered products as well as reduced wear of equipment ... 

Applications bottles, calendered products, medical, pipes and fittings, siding, windows, wire and cable ... 

Calendered products include sheet stock, belting, and friction and coated fabrics. A large proportion of sheet stock is later diefinished products such as gaskets, pads, and diaphragms. 

A modem calender is used to form a film or a sheet. Compounding operations are performed on auxiliary equipment which may be more complex than the calender itself Formulation and a repeatable compounding method frequently determine the quality of the calendered product. 

Recent developments in PP quality, together with some ecological credentials of the product and the process, are more and more attracting the attention of manufacturers of calendered products and machinery. The main problem in PP calendering is definitely the lack of experience... 

In respect to the calendered products, the most important differences between PP and PVC are density, rigidity, struchue and some ecological factors. Since PP has lower elasticity modulus than hard PVC, a PP film should have a higher thickness in order to reach the same rigidity level. 

Calender Production machine that uses a series of heated rollers to form thin plastic sheets while also producing a specific surface texture. 

Calendered products Calendered rubber sheets are widely used in various applications such as conveyer belts, bumpers, gaskets, roll coverings, brake linings, seals and tyre patches. 

SP rubbers find particular advantage when used in lightly loaded extruded or calendered products to the extent that even pure gum formulations can be processed as easily as heavily filled formulations. Particular benefits in extrusion are reduced die swell and faster throughput (Table 12). The effect of SP rubbers on vulcanizate properties is negligible. 

Calendering production of plastics sheet of high quality and to close tolerances by use of a roller mill comprising three or more rollers. Most products calendered at present are either PVC or ABS or a mixture of both. 

Calendered products such as film sheets and floor o)verings shower curtains, food covers, rainwear, handbags, coated fabrics, insulation for electrical cable and wire, phonograph records... 

S.4.2 Extruded Flexible Film and Sheet The resin viscosities used in extruded film and sheet are dependent on the application, but normally iT-values range from 60 to 71, with 66 being an average viscosity. Although resin gel content is critical in many extruded and calendered flexible film and sheet applications, extruded products are less tolerant to gels as a rule than calendered products. Since one can achieve sheet products of greater thickness with the extrusion process, differences in clarity between resins can become more important. 

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